Columnar arrangement of lumbar motoneurons innervating a pair of antagonistically acting leg muscles in the rat.

Various problems concerning the physiology of muscular units depend on the exact localization of motoneurons innervating antagonistically acting muscles. The present communication is focussed on the distribution of motoneurons innervating the gastrocnemius (GC) and tibialis anterior (TA) muscles. After injection of horseradish peroxidase (HRP) into these muscles and a survival time ensuring sufficient retrograde transport, the number of motoneurons, their segmental distribution, the mean area covered the labeled cells and the mean diameter of their somata were determined. After injections into the GC-muscle, 129 +/- 6 labeled perikarya were found, and following injections into the TA-muscle, 120 +/- 9 motoneurons were marked with HRP. The motoneurons of both muscles were distributed in spinal cord segments L4-5-6; however, the GC-neurons accumulated in segments L5-6 (approximately 94%) and the TA-neurons in L4-5 (approximately 95%). Although the motoneurons innervating both muscles were located in a rather similar area of the ventral column, i.e. its dorsolateral portion as judged from transverse sections, the GC-motoneurons were situated ventrolaterally to the TA-motoneurons. The measurement of the area of the somata and the mean soma diameter did not reveal any conspicuous differences between both pools of motoneurons. An unimodal distribution pattern of these parameters suggests a broad overlap in the size of alpha-, beta-, and gamma-motoneurons.